The present invention relates to a fuel cell in which an enzyme is immobilized on at least a cathode, a method for operating the same, and an electronic device using the fuel cell.
Fuel cells have a structure in which a cathode (oxidizing agent electrode) and an anode (fuel electrode) face each other with an electrolyte (proton conductor) therebetween. In conventional fuel cells, a fuel (hydrogen) supplied to the anode is oxidized and separated into electrons and protons (H+), the electrons are transferred to the anode, and the H+ are moved to the cathode through the electrolyte. On the cathode, the H+ react with oxygen supplied from the outside and electrons sent from the anode through an external circuit to produce water (H2O).
Thus, fuel cells are high-efficiency power-generating devices that directly convert chemical energy possessed by a fuel into electrical energy, and thus chemical energy possessed by energy from a fossil fuel such as natural gas, petroleum, or coal can be extracted as electrical energy regardless of the place or the time of use, and in addition, with a high conversion efficiency. Accordingly, hitherto, research and development of fuel cells for the application to a large-scale power generation or the like has been actively carried out. For example, a fuel cell was installed in a space shuttle, and it was demonstrated that water for the crew can be replenished together with generation of electric power and that the fuel cell is a clean power-generating device.
Furthermore, recently, fuel cells that exhibit a relatively low operation temperature range of room temperature to about 90° C., for example, solid polymer-type fuel cells, have been developed and attracted attention. Consequently, not only large-scale power generation applications but also applications of a fuel cell to a small system such as a power supply for powering automobiles or a portable power supply for a personal computer, a mobile device, or the like have been searched for.
As described above, a wide range of applications ranging from large-scale power generation to small-scale power generation can be expected for fuel cells, and the fuel cells have attracted a lot of attention as power-generating devices with a high efficiency. However, in fuel cells, since natural gas, petroleum, coal, or the like is usually used as a fuel by converting it into hydrogen gas with a reformer, limited resources are consumed, it is necessary to perform heating to a high temperature, and a catalyst composed of an expensive noble metal such as platinum (Pt) is necessary, thus causing various problems. In addition, even in the case where hydrogen gas or methanol is directly used as a fuel, care should be taken in the handling thereof.
Under these circumstances, the fact that the biological metabolism carried out in the living organism is a high-efficiency energy conversion mechanism has been focused and an application of this to a fuel cell has been proposed. The term “biological metabolism” used herein includes aspiration, photosynthesis, and the like carried out in microbial somatic cells. The biological metabolism combines the feature that the power generation efficiency is very high with the feature that a reaction proceeds under a mild condition at about room temperature.
For example, aspiration is a mechanism in which nutrients such as saccharides, fat, and proteins are taken into microbes or cells, the chemical energy thereof is converted into oxidation-reduction energy, i.e., electrical energy, by reducing nicotinamide adenine dinucleotide (NAD+) to reduced nicotinamide adenine dinucleotide (NADH) in a process of producing carbon dioxide (CO2) through a glycolytic pathway and a citric acid (TCA) cycle including a large number of enzyme reaction steps and, furthermore, in an electron transport system, the electrical energy of NADH is directly converted into the electrical energy of a proton gradient and, in addition, oxygen is reduced to produce water. The electrical energy obtained here produces adenosine triphosphate (ATP) from adenosine diphosphate (ADP) with ATP synthase, and this ATP is used for reactions that are necessary for growing microbes or cells. This energy conversion is carried out in the cytosol and mitochondria.
Also, photosynthesis is a mechanism in which water is oxidized to produce oxygen in a process of taking light energy and converting the light energy into electrical energy by reducing nicotinamide adenine dinucleotide phosphate (NADP+) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) through the electron transport system. This electrical energy takes CO2, is used for a carbon-fixation reaction, and is used for synthesis of carbohydrates.
As for a technology to use the above-described biological metabolism for a fuel cell, a microbial cell has been reported, in which electrical energy generated in microbes is taken out of the microbes through an electron mediator and the electron is transferred to an electrode to obtain a current (refer to, for example, Japanese Unexamined Patent Application Publication No. 2000-133297).
However, in microbes and cells, a large number of unnecessary reactions are present besides the target reaction such as conversion of chemical energy to electrical energy. Therefore, in the method described above, electrical energy is consumed in undesired reactions and a sufficient energy conversion efficiency is not achieved.
Under these circumstances, fuel cells (biofuel cells) in which only a desired reaction is conducted by using an enzyme have been proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-282124, Japanese Unexamined Patent Application Publication No. 2004-71559, Japanese Unexamined Patent Application Publication No. 2005-13210, Japanese Unexamined Patent Application Publication No. 2005-310613, Japanese Unexamined Patent Application Publication No. 2006-24555, Japanese Unexamined Patent Application Publication No. 2006-49215, Japanese Unexamined Patent Application Publication No. 2006-93090, Japanese Unexamined Patent Application Publication No. 2006-127957, Japanese Unexamined Patent Application Publication No. 2006-156354, Japanese Unexamined Patent Application Publication No. 2007-12281, Japanese Unexamined Patent Application Publication No. 2007-35437, and Japanese Unexamined Patent Application Publication No. 2007-87627). In these biofuel cells, a fuel is decomposed by an enzyme to separate into protons and electrons, and biofuel cells in which an alcohol such as methanol or ethanol or a monosaccharide such as glucose is used as the fuel have been developed.